In various types of applications the contents of an optically transparent article with curved walls, such as, for example, a cylindrical tube of glass or quartz, may be diagnosed or analyzed by inspection of radiation emitted from or transmitted through the contents of the article. In passing through the curved walls of the article, however, the path of the radiation is distorted by, e.g., refraction. Such refraction causes the path of the radiation passing through the contents of the article to differ from the intended and expected path. The refraction of the ray is more pronounced as the position of the ray approaches being tangential to the inner wall of the article. Such effects create problems in applications, such as, for example, the diagnosis of light emitted from discharge lamp arc tubes, and the diagnosis of laser beams transmitted through or scattered by arc discharges. Similar problems may exist in commercially-available analytical devices which involve passing a beam of radiation through a sample holder having curved walls such as in, e.g., atomic absorption spectrometers.
The problem of refraction has been particularly troublesome in laser diagnostic techniques which attempt measurements near the inner wall of a discharge lamp arc tube.
One approach for minimizing the refraction problem in arc discharge diagnostics involves moving the detector to intercept the ray along its emergent path. B. Weber, "Mapping of Mercury and Xenon Densities in High Pressure Lamps", 4th Int'l. Symp. on the Sci. and Techn. of Light Sources, Paper No. 1, Karlsruhe, F. R. Germany (April 1986).
This technique, however, is unsatisfactory for rays nearly tangential to the inner wall of the sample vessel. This technique is further not suitable for use in applications which require the light ray to follow an undeviated path through the sample. One example of an application with such requirement involves two intersecting laser beams which must be kept aligned inside the sample as one or both beams is scanned laterally relative to the sample.
The deviation of the light ray path through a sample is especially undesirable in techniques where light scattered from a light beam source or laser is to be detected as the light or laser beam position is scanned through the sample. See, for example, W. J. van den Hoek, Philips J. Res. 38, 188-213 (1983).